A solder mask (also called “solder resist”) is a coating used to protect (mask) certain areas of a printed circuit board during the soldering of connections to the circuit board. Because the solder mask covers most of the circuit board, it protects the circuitry and provides electrical insulation. Thus, only a small area of the circuit is exposed to solder. Therefore, as less solder is used, the likelihood of solder bridging over the circuit lines and other features is reduced. Also, there is less chance that contaminates will be transferred into the solder container from the circuit board. Also, the solder mask reduces dendritic growth of the copper elements on the circuit board. (The solder mask also isolates the PCB circuit traces from the external environment.)
A printed circuit board may be observed to fail during testing. Failure analysis engineers must make a determination of the reason for the failure. Because the failure may be the result of a short or open circuit in a metal trace on a layer of the printed circuit board beneath the solder mask, it may be necessary to remove the solder mask to complete the failure analysis.
A known technique for solder mask removal used by failure analysis engineers is polishing the solder mask until the defect is revealed. However, using the polishing method, it is very easy to cause artifacts and damage the circuit trace. Thus, it becomes difficult or impossible to determine whether a defect observed in the underlying circuit trace was present before the polishing, or caused by the polishing.
Other conventional techniques for solder mask removal include milling, grinding, microblasting, and chemical stripping.
Milling involves use of sharp cutters, so precision depth control is required. The milling system requires a microscope. The carbide cutters typically used are so sharp they will tend to pull into the coating and may penetrate into the board surface.
Grinding involves the use of a knife, scraper or pick by a skilled technician. No special setup is needed, but operator fatigue can be a disadvantage.
Microblasting involves propelling an abrasive material at the solder mask. The abrasive material blasts away the coating. This creates substantial friction and static charges. For circuit boards with static sensitive devices, the tool must be designed to eliminate potential ESD damage. Significant preparation time including masking is often needed. A thorough cleaning is required to flush away any blasting material from the circuit board. Operator skill and training are also required.
Chemical stripping involves application of a material like a paint stripper to the solder mask. Because the stripper is liquid, it is often hard to control. The most common chemical strippers can deteriorate the base material beneath the solder mask if exposed to the stripper for too long a time.
A failure analysis method that does not cause further damage to the underlying circuitry of the printed circuit board is desired.